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10 Posts
Discussion Starter · #1 ·
Hey Guys,

I'm back to pick some more of your brains as I am stumped...

98 Mustang V6 Convertible. approx 130,000 miles

Issue: MIL light is ON
-Checked my codes:
*P0174 System to Lean Bank 2
*H02S 2/1 Insufficient Switching Lean Mean
*02S Ckt Slow Response 2/1

So I figured it was an issue with an 02...

I pulled each O2 (one by one) and cleaned each one with carb cleaner, cleaned each connector and harness and reinstalled, but flip flopped the pre-cat manifold O2's to see if the error would follow the O2 sensor...

for once the error (after buzzing around) flipped to bank 1 sensor 1. I was very happy even though I had to melt my hand off to pull the O2 out of the now 300 degree manifold.

Installed a new sensor, drove around a bit after clearing the codes and no engine light... but to my horror three-four new codes awaited me in pending codes to be solidified on the next drive cycle.

Bank 1 Lean
Bank 2 Lean
O2 Ckt Malfunction 2/1 AND 1/1

Now I was puzzled... I knew it couldn't be an O2 issue... but maybe it needed to be driven a bit longer.

About a week and a half later same issues... even after clearing the codes I receive O2 errors, lean errors, and malfunctions on the heated O2's...

I found this odd because the exhaust is coated (excessively) in black soot. after one consecutive startup and driven for about 4 miles, the exhaust has a thick layer of carbon (indicates running rich). I figure this to be the PCM/EEC compensating for the lean error/dumping gas while O2's are heating up (malfunctioning).

So I switched gears to the fuel system. Maybe the issue was with getting fuel to the injectors. I decided to replace the fuel pressure regulator to no avail (same issues).

I also have cleaned my MAF sensor + IAT sensor and the errors persist.

When I first bought the car (few months ago), I gave her a tune up and installed:
-new plugs
-new wires
-cleaned fuel injectors / tested them
-new upper intake gasket
-cleaned out intake pleneum
-throttle body gasket
-thermostat + gasket
-new fuel filter
-flushed tranny
-fuel treatment
-checked for cracked/damaged vacuum lines

I am now stumped...

Upon more research of this issue, I've found numerous "culprits"...

Currently I am investigating:
-EGR Valve
-EGR Regulator Solenoid
-EEC/PCM Fuse?
-DPFE sensor

Some people also say monitor the fuel pressure at the rails, but unfortunately I am unable to get my hands on a pressure gauge for a bit. I'm ruling out the pump/pressure for now since the pump seems/is strong and there is no sputtering involved...

I'd be grateful if someone could point me in a direction... I'm growing agitated xD!


6,859 Posts
The key to your problem maybe the O2 malfunction code. Start first by checking the fuses.

Next check the wiring harness for problems. The typical thing that happens is the O2 pigtails touches the hot exhaust and melts the harness. This could also be something as simple as the connector not fully inserted.

Note, all 4 O2 sensors share the same heater circuit. So the wiring problem does not have to be at the front O2 sensor.

How many miles? I personally would not clean an O2 sensor with 100k+ miles on it. Replace the front ones if over 100K. Then you are working from known good sensors.

Are there any exhaust leaks?

How did you clean the MAF? If you used carb/choke cleaner, this has likely ruined it. Use only a product designed for the job.

Once all of the fuses have been confirmed, let's go from there. It just doesn't make sense to write about all of the things it could be when the basics haven't been confirmed.

Note, one thing this could be is someone has switched to larger fuel injectors and restored a factory tune. This would account for why it's running chronically rich.

10 Posts
Discussion Starter · #3 ·
Hey wmburns!

Uh... 130k on her... I already replaced one of the front O2's and I'm still seeing the same error from that one... and the other front heated O2... all connectors have been cleaned and plugged/unplugged numerous times to make sure they're snug.

The pigtail on the one that I thought was bad was definately singed... but not shorted... but I replaced it anyway...

Still no luck... I cleaned the MAF with no residue electronics cleaner... works fine / watched the output via my scanner tool.

The reason I posted with what "seems" to be no confirmation on the basics is because I've gone over them 9,000 times... I've pulled out virtually every fuse, checked their continuity, and sprayed the living crap out of the fuse block with electronic cleaner, then reinstalled them snuggly... I do know my dad had an issue with his jeep when he pushed fuses in, the actuall harness would drop down/not connect with the fuse... and I've checked for this xD

There are no exhaust leaks prior to or after the cats. The only one I am aware of is one near the muffler where my welding/brazing skills kinda failed xD It's very small (pinhole) and it had this issue prior to doing any exhaust work.

Fuel injectors are stock (the guy I purchased it from didn't use any tunes /it was his wifes car). I don't really trust most peoples word, so I checked them either way and cleaned.

I'll go out and double check the fuses today. If it was a fuse, the PCM/EEC should be acting up also I would assume.


6,859 Posts
Please post the full DTC codes.

Having access to an ODB2 scanner that is able to monitor operational data is likely to make short work of this. What we really need to know is what the MAF sensor is telling the PCM and what the Long term fuel trim values are doing.

Because you haven't posted the DTC's, I don't know if the malifunction is in the O2 heater or in the O2 return line. Regardless, it may be a good idea to inspect the main PCM connector for evidence of water damage.

10 Posts
Discussion Starter · #5 ·
Thanks for your help!


I know this is opening an old thread, but I would like to resolve this thread for anyone who may have a similar issue.

I attempted to clean the front O2 sensors, but still had the lean error DTC.

So I went out and bought 2 new front O2 sensors and screwed them in. Issue resolved!

I need to learn to be less stubborn and buy replacement sensors when they go bad... It's hard to clean a bad sensor :D

I believe my O2 sensors were plugged from excessive oil burning in the cylinders due to blow-by. See my other thread for info on that!

Thanks again!
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